Glass antenna system for automobile

ABSTRACT

A glass antenna system for an automobile comprises a T-shaped or pi-shaped dynamic impedance matching circuit. Another system comprises an integrated circuit consisting of T-shaped FM and AM matching circuits each connected to a common cable. The T-shaped or pi-shaped matching circuit comprises at least two varactor diodes each applied to a channel selection voltage to match the FM antenna impedance with that of the FM receiver including that of the transmission cable.

This application is a divisional of application Ser. No. 08/459,533,filed Jun. 2, 1995, now U.S. Pat. No. 5,602,558, which in turn is acontinuation of application Ser. No. 08/269,632, filed Jul. 1, 1994, nowabandoned, which in turn is a continuation of application Ser. No.07/857,377, filed Mar. 25, 1992, now abandoned.

BACKGROUND OF TEE INVENTION

1. Field of the Invention

The present invention relates to a glass antenna system for anautomobile in which the impedance of a glass antenna provided on theautomobile is matched for that of a car radio or receiver connectedthrough a transmission cable. It further relates to a glass antennasystem comprising FM and AM antennas each provided on the window glassof the automobile, FM and AM dynamic matching circuits each connected tothe corresponding antenna and a common transmission cable.

2. Description of the Prior Art

The inventors proposed with Japanese patent application laid open No.3-49402 (1991) and utility model application No. 63-143591 (1988) aglass antenna system for an automobile without any preamplifier. Theglass antenna 2 as shown schematically in FIG. 1 is connected to aconventional dynamic matching circuit 3 provided on a rear glass 1 andhaving its circuit factor corresponding to the selected frequency of thereceiver 6. On the rear glass 1, a plurality of heaters or wires 4 isprinted as an AM antenna 4, while the FM antenna 2 is printed on theupper portion of the most significant heater 4. When the FM antenna 2 isprinted on the rear glass 1, its size or dimension is limited even ifpatterns of the antenna are designed. Accordingly, the resistancecomponent of the FM antenna 2 is often changed between the higher andlower values than that of a transmission cable 5.

FIG. 2 generally shows the active matching circuit 3 comprising acapacitor 10 connected between an input terminal 11 and an input node12, and a coil 13 connected between the input node 12 and an output nodeor terminal 14. The input terminal 11 is also connected to the FMantenna 2. The matching circuit 3 further comprises a DC cutoffcapacitor 15 having relatively high capacitance (e.g. 0.1 nF) andconnected between the input node 12 and a relay node 16, and a varactordiode 17 connected between the relay node 16 and output node 14. Aresistor 18 and capacitor 19 each connected to ground are connected tothe relay node 16 and the terminal 14 respectively.

The anode of the varactor 17 is grounded through the resistor 18.Therefore, the coil 13, capacitor 15 and varactor 17 constitutes aresonance circuit tuned to a predetermined frequency, to control theresonance frequency by a voltage SE applying to a cathode of thevaractor 17 thorough the cable 5.

FIG. 3 shows a basic principle circuit of the conventional dynamicmatching circuit shown in FIG. 2. As apparent from an impedance matchingtheory, the circuit shown in FIG. 3 does not have a matching functionwhen the resistance component Rx of the antenna impedance is higher thanthat RL of the load impedance. The conventional matching circuit cannotbe applied to such glass antenna patterns in case of the Rx more thanthe RL. The impedance of the load side must have only the resistancecomponent.

When the characteristic impedance of the cable 5 is different from theinput impedance of the receiver 6, the impedance viewed from an end ofthe cable 5, that is, the matching circuit to the receiver includesmarginal or reactance component. The gain of the receiving system isreduced because of mismatching between the impedance of the antenna andthat of the receiver 6 connected through the cable 5.

As described above, the conventional receiving system having the dynamicmatching circuit connected to the glass antenna must fulfill thecondition that the resistance component of the antenna impedance issmaller than that of the load impedance of the receiver 6 viewed throughthe cable 5. Because the antenna impedance range being available islimited and its size of the antenna pattern is then limited, adjustmentsof its directivity and gain are also limited. Therefore, it isimpossible to obtain a suitable glass antenna pattern having desiredgain and directivity.

The conventional receiving system has reduced receiving system gainsbecause the impedance of the glass antenna side does not adequatelymatch that of the load side of the receiver in case that thecharacteristic impedance of the cable is different from the inputimpedance of the receiver each other.

FIG. 4 shows another prior art of the antenna system havingpreamplifiers 7 and 8. The same numerals are denoted in partscorresponding to the those of FIG. 1. The FM preamplifier 7 is connectedbetween the FM antenna 2 and the cable 5 while the AM preamplifier 8 isconnected between the AM antenna 4 and the cable 5.

In such a conventional receiving system, the FM preamplifier 7 is activewhen an AM signal is received by the receiver 6. The FM preamplifier 7has a wide frequency characteristic over the FM broadcast band and doesnot have a sharp selectivity against a predetermined electric wave to bedesired. When the automobile is moved to the area in which strongelectric fields of radio waves radiated by at least two FM broadcaststations are overlapped and their frequency's difference is enteredwithin a certain frequency of the AM frequency band, by theintermodulations among those FM radio waves in the FM preamplifier 7 andthe receiver 6, undesirable AM signals are generated.

Accordingly, in the conventional receiving system for the automobilehaving preamplifiers, when the automobile is moved to the area thatelectric fields of radio waves issued by two or more of FM broadcaststations are strong and their frequency's difference is entered withinthe AM frequency band, intermodulation noises by those FM radio wavesare generated upon receiving the electric wave from the AM station.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an improveddynamic matching circuit for the glass antenna of the automobile. In theimproved matching circuit, the impedance of the receiver connectedthrough the cable is matched with that of the glass antenna even if itsresistance component of the antenna impedance is changed between higherand lower than that of the load in the specific frequency range forexample FM broadcast band. The load side is defined as viewed to thereceiver from the end of the cable.

It is another object of the invention to provide the improved dynamicmatching circuit for the glass antenna system of the automobile withoutany preamplifier. Many types of glass antenna patterns are possible byrealizing the impedance matching between the glass antenna and receiveror load over the specific frequency range for example FM broadcast band.

It is further object to provide the glass antenna system of theautomobile in which the intermodulation noise invaded upon receiving theAM broadcast in the area present two or more of intensive FM broadcastwaves having difference of those frequencies being within the AMfrequency band.

According to the present invention, the dynamic matching circuit for theglass antenna for the automobile comprises: a main matching circuitconnected to an antenna provided on a window glass of the automobile;and at least one auxiliary matching circuit operatively connectedbetween said main matching circuit and a transmission cable, forcompensating said main matching circuit.

In the present invention, said main and auxiliary matching circuits arecombined to provide the improved matching circuit. The improved matchingcircuit comprises an antenna reactance cancel circuit for cancelling thereactance of said window glass antenna, a load reactance cancel circuitfor cancelling the reactance of said receiver and a conversion circuitfor converting the impedance of said antenna after cancelling saidreactance of said antenna by said antenna reactance cancel circuit tothat of said load after cancelling said reactance of said load by saidload cancel circuit.

Further, according to the present invention, the antenna system for theautomobile comprises FM and AM antennas each provided on the window rearglass of the automobile, FM and AM dynamic matching circuits eachconnected to the corresponding antenna, and a common cable connected tosaid FM and AM dynamic matching circuits. The dynamic matching circuitcomprises at least two variable reactance element to which a DCfrequency selection signal is applied through the cable from saidreceiver.

Accordingly, the AM dynamic matching circuit is active based on the AMfrequency selection signal upon receiving the AM broadcast.Simultaneously, the FM dynamic matching circuit is active based on theAM frequency selection signal. The intermodulation noise by the FMelectric wave upon receiving the AM broadcast can be reduced because theactive FM dynamic matching circuit has selectivity for FM band signalsand limits the frequency range of the interference FM wave that inducesthe intermodulation noise.

The above, and other objects, features and advantages of the inventionwill be apparent in the following detailed description of illustrativeembodiments of the invention which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a conventional receiving system showing arelation among an antenna on a rear glass of an automobile, a receiverand a dynamic matching circuit;

FIG. 2 is a conventional circuit diagram of the dynamic matchingcircuit;

FIG. 3 shows a basic principle circuit of the dynamic matching circuitshown in FIG. 2;

FIG. 4 shows another prior art of the antenna system havingpreamplifiers;

FIG. 5 is a circuit showing an embodiment of the T-shaped dynamicmatching circuit according to the present invention;

FIG. 6 is a circuit showing an embodiment of a multi-stage or pi-shapeddynamic matching circuit according to the present invention;

FIG. 7 is a circuit diagram showing another embodiment of the T-shapeddynamic impedance matching circuit according to the present invention;

FIG. 8 is an equivalent receiving system using the T-shaped dynamicmatching circuit of FIG. 5;

FIG. 9 is a block diagram in which an operation of the receiving systemof FIG. 8 is resolved into basic functions;

FIG. 10 is a circuit showing an embodiment of a H-shaped dynamicmatching circuit according to the present invention modified from thedual unbalanced T-shaped dynamic matching circuits each shown in FIG. 5to its balanced configuration;

FIG. 11 is another embodiment of the antenna system for the automobilecomprising FM and AM dynamic matching circuits according to the presentinvention;

FIG. 12 is a circuit showing an embodiment of the FM and AM dynamicmatching circuits of FIG. 11;

FIG. 13 is a circuit showing another embodiment of the FM and AM dynamicmatching circuits of FIG. 11;

FIG. 14 is a graph showing frequency to gain characteristics between thepresent FM dynamic matching circuit of FIGS. 12 and 13 according to thepresent invention and the conventional preamplifier of FIG. 4;

FIG. 15 is a schematic view of an embodiment of a window glass antennaaccording to the present invention; and

FIG. 16 is a schematic view of another embodiment of a window glassantenna according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 5 shows a first embodiment of the T-shaped dynamic matching circuitaccording to the present invention. The T-shaped dynamic matchingcircuit 30 comprises a main matching circuit 31 connected to an antenna2 shown in FIG. 1, an auxiliary matching circuit 32 operativelyconnected between said main matching circuit 31 and the transmissioncable 5 shown in FIG. 1. Referring to FIG. 6, another dynamic matchingcircuit having two of the auxiliary matching circuits 31 cascadedtogether is shown. In FIG. 5, since the construction of the mainmatching circuit 31 is identical to that of the conventional dynamicmatching circuit 3 except for a 1-picofarad capacitor 20 paralleled tothe varactor diode 17, its explanation is omitted.

On the other hand, the auxiliary matching circuit 32 comprises a secondcoil 21 connected between another input node 14 and another relay node22. The another input node 14 is common to the output node 14 of themain matching circuit 31. The auxiliary circuit 32 comprises a secondvaractor diode 23 connected between the input node 14 and another outputnode 24, a second DC cutoff capacitor 25 connected between the nodes 22and 24 to provide a second resonance circuit, and resistors 26 and 27for relaying a capacitance control voltage SE to the varactor diodes 17and 23. The control voltage SE issued from the receiver 6 is applied tothe varactor 23 through the cable 5 and a load-side terminal 28, whilethe FM signal from the antenna 2 passed through a coupling capacitor 29connected between the output node 24 and terminal 28. The resistor 26 istherefore connected between the terminal 28 and relay node 22 while theresistor 27 is connected between the output node 24 and ground.

Accordingly, the T-shaped dynamic matching circuit 30 has variablereactance circuits including its central capacitor 19, and varactordiodes 17 and 23 each disposed on right and left branches. Theirreactance is controlled by the capacitance of the varactor diodes withthe common frequency control voltage supplied from the receiver 6.

In the embodiment of FIG. 5, values of the capacitors 10, 19 and 29 maybe set to be 6, 10 and 6 picofarads respectively, and values of thecapacitors 15 and 25 may be set to be 100 nanofarads. The resistors 18,26 and 27 may have 100 kilo-ohms or higher than the 100 kilo-ohmsbecause reverse bias voltage is applied to the varactor diodes 17 and 23to adjust their capacitance without a certain power consumption. Thecoils 13 and 21 may be 200 nanohenry. The varactor diodes 17 and 23 maybe a type No. 1SV153.

In the multi-stage dynamic matching circuit 30 comprising a plurality ofauxiliary matching circuits 32 and 33 as shown in FIG. 6, theconstruction of the first auxiliary matching circuit 32 is identical tothat of the second auxiliary matching circuit 33 except for anadditional capacitor 34 connected between the input node 14 and ground.This grounded capacitor 34 has a function substantially identical tothat of the capacitor 19. The coupling capacitor 29 of the firstauxiliary matching circuit 32 is connected between its output node 24and the input node 14, of the second auxiliary matching circuit 33. Inthe multi-stage dynamic matching circuit 30 according to the presentinvention, a third or more of auxiliary matching circuits 32 may beemployed.

FIG. 7 shows another embodiment of the T-shaped dynamic matching circuit30. In the drawing, an input terminal 11 is connected to the glassantenna 2 while an output terminal 28 is connected to the transmissioncable 5 as shown in FIG. 4. The patching circuit 30 comprises a centralcapacitor 19 connected between a first node 14 and ground, and first andsecond variable reactance circuits as right and left branchconfigurations. The first variable reactance circuit comprises acoupling capacitor 10 connected between the input terminal 11 and asecond node 16. A coil 13 and a capacitor 20 are connected between thefirst and second nodes 14 and 16. Anodes of cathode common varactordiodes 17a and 17b are connected to the second and first nodes 16 and 14respectively. The common cathode of the varactor diodes 17a and 17b isconnected to the output terminal 28 through a resistor 26a. The secondvariable reactance circuit comprises a coupling capacitor 29 connectedbetween the output terminal 28 and a third node 24. Another coil 21 anda capacitor 20b are connected between the first and third nodes 14 and24. Anodes of cathode common varactor diodes 23a and 23b are connectedto the first and third nodes 14 and 24 respectively. The common cathodeof the varactor diodes 23a and 23b is connected to the output terminal28 through a resistor 26b.

Resistors 19b, 18 and 27 each connected to ground are connected to thefirst, second and third nodes 14, 16 and 24 respectively to apply a biasvoltage to the varactor diodes 17a, 17b, 23a and 23b through theresistors 26a and 26b.

In this embodiment, to set the impedance viewed from the cable 5 to be75 ohms, the saw-toothed voltage sweep is applied to the common cathodesof the varactor diodes 17a, 17b, 23a and 23b as well as the innervaractor diode in the FM receiver. The sweep has the voltage rangecorresponding to the predetermined FM frequency band. When anappropriate FM channel at a given frequency is selected by the FMreceiver, the appropriate voltage is applied to the matching circuit tomatch the FM antenna with the FM receiver through the transmission cableat the given frequency. Reactance component of the antenna at the givenfrequency is therefore cancelled with controlled capacitance of thevaractor diodes.

For example, the resistors 18, 19b, 27, 26a and 26b may have 100kilo-ohms or more. The capacitor 10 may have 5 to 50 picofarads (e.g.,30 picofarads), while the capacitor 29 have 5 to 50 picofarads (e.g., 10picofarads). The capacitor 19 may have 5 to 50 picofarads (e.g., 10picofarads), while the capacitors 20 and 20b have 0 to 50 picofarads(e.g., 2 picofarads). The coils 13 and 21 may have 100 to 300 nanohenry(e.g., 200 nanohenry). The varactor diodes 17a, 17b, 23a and 23b may bea type No. 1SV161.

Particularly, the matching circuit as shown in FIG. 7 can effectivelyreduce a noise induced in a strong or intensive electric field ofbroadcast electric waves.

In such T-shaped or multistage dynamic matching circuit 30, a DC voltageSE of the frequency selection signal from the receiver 6 is appliedthrough a core line of the coaxial cable 5. In the embodiment of FIG. 5,since the characteristic impedance of the cable 5 is 122 ohms and theinput impedance of the receiver 6 or load may be 75 ohms, the impedanceviewed to the receiver or load side through the cable 5 containsimaginary component and its value is changed in response to thefrequency.

FIG. 8 shows an equivalent circuit of the receiving system comprisingthe T-shaped dynamic matching circuit of FIG. 5, the rear glass antenna2 and the receiver 6. The resistance and reactance components of theglass antenna 35 at the specific frequency are assumed to be Rx and Xxrespectively. The source of the electric wave signal is assumed to be36. The resistance and reactance components of the black box 37 viewedto the receiver load side through the transmission cable glass areassumed to be RL and XL respectively.

FIG. 9 is a block diagram in which an operation of the equivalentreceiving system of FIG. 8 is resolved into basic functions. In drawing,same numerals are denoted to parts corresponding to the those of FIG. 8.When Rx<RL, it composes an antenna reactance cancel circuit 38 forcancelling the reactance component of the antenna by the variablereactance circuit on the left side or antenna side branch as shown inFIG. 9a. The cancel circuit 38 is provided with he resonance circuitconsisting of the coil 13, capacitors 15 and 20 and varactor diode 17.An impedance converter or circuit converts to RL the remaining Lcomponent after cancelling the antenna reactance and the resistancecomponent when the capacitor 19 is viewed from output or load side toantenna side.

While, it composes a load reactance cancel circuit 39 for cancelling thereactance component of the impedance viewed to the receiver or load sidethrough the cable by the variable reactance circuit on the right side orcable side branch, that is, by another resonance circuit consisting ofthe coil 21, capacitor 25 and varactor diode 23. Therefore, impedancebetween the antenna side and receiver side are adequately matched.

On the contrary, when Rx>RL, it composes a first reactance cancelcircuit 38' for cancelling the reactance component of the antenna by thevariable reactance circuit on the left side or antenna side branch asshown in FIG. 9b, that is, by the resonance circuit consisting of thecoil 13, capacitors 15 and 20 and varactor diode 17.

It further comprises a second reactance cancel circuit 39' forcancelling the reactance component of the impedance viewed to thereceiver or load side through the cable by the variable reactancecircuit on the right side or cable side branch, that is, by anotherresonance circuit consisting of the coil 21, capacitor 25 and varactordiode 23. Another impedance converter or circuit converts to RL theremaining L component after cancelling the cable reactance and theresistance component when the capacitor 19 is viewed from output side toantenna side. In this case, the impedance between the antenna side andthe receiver side is adequately matched. Again, when Rx>RL, theimpedance between the antenna side and receiver side is adequatelymatched by having the functions of the branch circuits reversed to thoseof Rx<RL case vice versa.

FIG. 10 shows an embodiment of a balanced dynamic matching circuitmodified from the dual unbalanced T-shaped dynamic matching circuitseach shown in FIG. 5. In the FIG. 10, a balanced antenna is connected tothe input terminals 11 of two main matching circuits 31, 31 except forthe capacitor 19 connected between the respective output nodes 14, 14 ofthe main matching circuits 30, 30. The output nodes of the main matchingcircuits 31, 31 are connected to the input nodes 14 of the auxiliarymatching circuits 41, 42. The output nodes 24, 24 of the auxiliarymatching circuits 41, 42 are connected to primary windings of a 4 to 1Balance to Unbalance transformer 43 through the respective couplingcapacitors 29, 29.

This transformer 43 converts the balance impedance of for example 300ohms to 75 ohms and its output is connected to the transmission orcoaxial cable 5 of for example 75 ohms. While, capacitance adjustingvoltage SE is applied to the respective varactor diodes 23, 23, 17, 17of the main and auxiliary matching circuits 31, 31, 41, 42 through thecore conductor of the cable 5, one of the primary winding and resistors26, 26. In the auxiliary matching circuits 42 having its outputconnected to ground through another primary winding, the resistor 26 isconnected to the output side of the capacitor 29 of the circuit 41. Thesecondary winding 44 opposed to the grounded primary winding isconnected to a high pass capacitor 45 connected to the output terminalto prevent the control voltage SE flowing through the core conductor ofthe cable 5 from shorting to ground.

As described above, the dynamic matching circuit according to theinvention comprises a main matching circuit connected to an antennaprovided on a window glass of the automobile. It further comprises atleast one auxiliary matching circuit operatively connected between saidmain matching circuit and a transmission cable, for compensating saidmain matching circuit. When the resistance component of the antennaimpedance is more than that of the load impedance, the auxiliarymatching circuit can compensate or assist the main matching circuit evenif the main matching circuit can not be matched between the antenna andload side. Therefore, the antenna can be matched with the load sideregardless of size between resistance component of the antenna impedanceand that of the load impedance. Further, in the receiving system, thefall or reduction of the gain is occurred by impedance mismatchingbetween the antenna and load including the cable.

In another embodiment of the dynamic matching circuit according to thepresent invention, said main and auxiliary matching circuits arecombined to provide the T-shaped matching circuit so that variablereactance circuits are constituted on right and left branches.Therefore, respite of simple circuit construction, as a role of thebranches can be exchanged according to the size of resistance componentsof their impedance between the antenna and load sides, a superiorimpedance matching between the antenna and load can be performedregardless the sizes thereof.

FIG. 11 shows a second embodiment of the glass antenna for theautomobile according to the present invention. The same numerals aredenoted in parts corresponding to the those of FIGS. 1 or 5. The antennasystem comprises an AM antenna consisting of a plurality of defoggingheaters 4 each providing on the rear glass 1, a reverse T-shaped FMantenna 2 provided on the rear glass 1 above the heaters 4, an FMdynamic matching circuit 30 connected to the FM antenna 2 through theinput terminal 11 and an AM dynamic matching circuit 50 connected to theAM antenna 4. The outputs of the FM and AM dynamic matching circuits 30and 50 are combined to connect to the receiver 6 through the commontransmission cable 5. The FM and AM dynamic matching circuits 30 and 50are integrated on a common substrate that is mounted on the rear glass1.

The FM dynamic matching circuit 30 comprises varactor diodes 17 and 23to which a DC frequency selection signal is applied from the receiver 6through the cable 5 as shown in FIG. 12. The AM matching circuit 50comprises a third varactor diode 51 to which the DC frequency selectionsignal is also applied.

The FM dynamic matching circuit 30 is T-shaped configuration as shown inFIG. 12 and comprises the capacitor 10 connected to the FM antenna 2shown in FIG. 11 through the input terminal 11. Another end of thecapacitor 10 is connected to the coil 13 and capacitor 15. The varactor17 is connected between another ends of the coil 13 and capacitor 15 toprovide a first resonance circuit. The anode of the varactor 17 isgrounded through the resistor 18 and paralleled to the, for example, 1picofarad capacitor 20 between its anode and cathode.

Another end of the coil 13 is grounded through the capacitor 19 andconnected to the second coil 21 and a cathode of the second varactor 23.The capacitor 25 is connected between another end of the second coil 21and anode of the second varactor 23 to provide a second resonancecircuit. The anode of the varactor 23 is connected to the cable 5through the capacitor 29 and the load side terminal 28. The resistor 26is connected between the load side terminal 28 and the another end ofthe second coil 21 to bypass the capacitors 25 and 29 for DC supplying.The anode of the varactor 23 is grounded through the resistor 27.

Therefore, the frequency selection signal applied to the load sideterminal 28 is supplied to varactors 17 and 23. A capacitor 49 havingfor example 1 picofarad is connected between the anode and cathode ofthe varactor 23.

On the contrast, the AM dynamic matching circuit 50 comprises acapacitor 53 connected to the AM antenna 4 shown in FIG. 11 through theinput terminal 52. Another end of the capacitor 53 is connected to athird coil 54. Another end of the third coil 54 is connected to acapacitor 55 having its relatively high capacitance and a fourth coil56. The third varactor 51 is connected between the capacitor 55 and thecoil 56 to provide a third resonance circuit.

The anode of the varactor 51 is grounded through a resistor 57. Thecathode of the varactor 51 is connected to a fifth coil 59 through acapacitor 58 and in turn connected to the load side terminal 28 throughanother capacitor 60. A resistor 61 is connected between the load sideterminal 28 and the cathode of the varactor 51 to bypass the capacitors58 and 60 to apply the frequency selection signal to the varactor 51.

FIG. 13 shows a second embodiment of the FM and AM dynamic matchingcircuits 64 and 66 as an integrated circuit. In FIG. 13, memberscorresponding to those of FIG. 12 denote same numerals. The FM dynamicmatching circuit 64 is different from the FM dynamic matching circuit 30as shown in FIG. 12 in that the capacitors 20 and 49 are omitted.

The AM dynamic matching circuit 66 is simplified compared to the AMdynamic matching circuit 50 as shown in FIG. 12. The capacitors 58 and60 are omitted as well as the resistor 61 to access the AM signal and DCfrequency selection signal through the coil 59. A Compensating capacitor68 having its several picofarads is connected between the anode andcathode of the varactor 51. The frequency selection signal is thenapplied to the varactor 51 through the coil 59.

In the embodiments of FIGS. 12 and 13, values of the capacitors 10, 19and 29 may be set to be 6, 10 and 6 picofarads respectively, and valuesof the capacitors 15 and 25 are 100 nanofarads. The resistors 18, 26 and27 may have 100 kilo-ohms or higher than the 100 kilo-ohms becausereverse bias voltage is applied to the varactor diodes 17 and 23 toadjust their capacitance without a certain power consumption. The coils13 and 21 may be 200 nanohenry. The varactor diodes 17 and 23 may be atype No. 1SV153. The varactor diode 51 may be another type No. KV1450.

The FM /AM integrated dynamic matching circuits 30 and 50 as shown inFIGS. 12 and 13 are mounted on the rear glass 1 and their circuitconstants are changed by the frequency selection signal from thereceiver 6. The integrated dynamic matching circuits each mounted on therear glass 1 are controlled by the frequency selection signal or channelselection voltage in the receiver, which is supplied from it through thetransmission cable 5.

The channel selection voltage is applied to both of the FM and AMdynamic matching circuits 30 and 50 upon selecting an AM or FM broadcastor channel station. The AM channel selection voltage for the AM dynamicmatching circuit 50 is also applied to the FM dynamic matching circuit30 upon selecting the AM broadcast or channel station. FIGS. 14a, 14band 14c show frequency versus gain characteristic by using theconventional preamplifier as shown in FIG. 4, or the present FM dynamicmatching circuit 30 on which the specific frequency f0 within the FMband is matched. In the FIG. 14a, the FM gain characteristic of theFM/AM integrated dynamic matching circuits according to the presentinvention has a relatively narrow or peak selectivity at the f0 whilethe conventional receiving system having the preamplifier has generallya flat or monotonic gain characteristic.

The FM matching frequency of the FM dynamic matching circuit is assumedto be f0 when the AM channel selection voltage is applied to it toreceive a certain AM channel. Two FM electric waves possibly giveninterferences to the certain AM channel upon receiving are assumed to befx and fy. The AM matching frequency of the AM dynamic matching circuitis assumed to be fa where fa=|fx-fy|. In case of using the FM/AMintegrated dynamic matching circuits according to the present invention,when the fx and fy are remote from the f0 as shown in FIG. 14b, theintermodulation noise is reduced sufficiently upon receiving the certainAM channel because the fx and fy are attenuated sufficiently uponpassing through the FM dynamic matching circuit.

Similarly, even if the fx or fy is close to the f0 as shown in FIG. 14c,the intermodulation noise is also reduced relatively upon receiving thecertain AM channel because the reining fy or fx is attenuated relativelyupon passing through the FM dynamic matching circuit. Namely, in case ofusing the FM/AM integrated dynamic matching circuits according to thepresent invention, if the frequency with the pair of the FM interferencechannels fx and fy is remote from the f0, the intermodulation noise isreduced sufficiently, or close to the f0, the intermodulation noise isreduced relatively. In either case, because the FM dynamic matchingcircuit has a relatively narrow frequency selectivity having a peak atthe specific FM frequency with respect to the specific AM selectionvoltage and no non-linear amplifier element is employed in the circuit,it is difficult to induce the intermodulation noise by the FMinterference channels or stations. It is possible to modify thecondition of the fx and fy as shown in FIG. 14c by shifting to anotherFM matching frequency f1 the matching frequency f0 of the FM dynamicmatching circuit upon applying the specific AM channel selectionvoltage, by modifying circuit constants of the FM or AM dynamic matchingcircuit. In the embodiments shown in FIGS. 12 and 13, although thecommon channel selection signal is applied to both of the FM and AMdynamic matching circuits, change of the frequency may be ascended ordescended against the ascending selection voltage. For example, the FMdynamic matching circuit may match at the frequency of 90 MHz while theAM dynamic matching circuit may match at the frequency of 535 KHz whenthe selection voltage is zero volt. The FM dynamic matching circuit maymatch at the frequency of 76 MHz while the AM dynamic matching circuitmay match at the frequency of 1605 KHz when the selection voltage is 10volts.

In the conventional receiving system having the FM preamplifier 7 shownin FIG. 4, because the frequency versus gain characteristic of itspreamplifier is substantially flat shown in FIG. 14, the FM signal isnot attenuated where the pair of the FM interference channels fx and fyis within the FM band. As the preamplifier 7 has non-linear gain, theintermodulation noise is generated upon receiving FM interferencechannels to produce higher intermodulation noise.

Therefore, the intermodulation noise by the FM stations upon receivingthe AM signal is reduced compared to that of the conventional receivingsystems as shown in FIG. 4, by providing the FM dynamic matching circuithaving its circuit constants in response to the frequency selectionsignal to the FM glass antenna according to the present invention asshown in FIGS. 12 and 13.

The FM and AM dynamic matching circuits as shown in FIGS. 12 and 13 arecontrolled by the common channel selection voltage. Alternatively, theFM and AM dynamic matching circuits may have a different voltage rangesfor make them matching within respective frequency bands. The constantsof the matching circuits are determined so that the varactor diodes 17and 23 receive a first voltage range of the frequency selection signaleffectively to match within the FM frequency band while the varactordiode 51 receives a second voltage range of the frequency selectionsignal effectively to match within the AM frequency band.

In this case, the capacitance of the capacitors and type number of thevaractors are determined so that the FM dynamic matching circuit ismatched with the range of zero to 10.0 volts in the FM frequencybandwidth of 76 to 90 MHz while the AM dynamic matching circuit ismatched with the range of 10.1 to 20.0 volts in the AM frequencybandwidth. Then, the FM dynamic matching circuit does not match withinthe FM band and then the FM signal passing through the FM dynamicmatching circuit is greatly attenuated when the specific voltage withinthe range of 10.1 to 20.0 volts is applied to the AM dynamic matchingcircuit. The first voltage range may partially include the secondvoltage range.

It is apparent to employ balanced dynamic matching circuits though FIGS.12 and 13 show unbalanced dynamic matching circuits. The presentinvention is not only applied with the unbalanced antenna system butalso with the balanced antenna system. FIGS. 15 and 16 show embodimentsin which a plurality of heaters 4 applied or printed on the rear glass 1employs both as an AM antenna 4 and FM subsidiary antenna 4. The reverseT-shaped FM main antenna 2 is applied or printed on the rear glass 1above the heaters 4.

The FM and AM signals from the AM/FM antenna 4 as shown in FIG. 15 iscommonly inputted to the FM /AM integrated dynamic matching circuit 71.The AM/FM antenna 4 as shown in FIG. 16 is different from that of FIG.15 in which power buses 72 and 73 each connected to heaters 4 areparalleled to a lateral T-shaped FM subsidiary pattern 74 to receive anFM subsidiary signal to the FM dynamic matching circuit, and an AMsignal from the bus 72 is inputted directly to the AM dynamic matchingcircuit.

Accordingly, in the FM/AM integrated dynamic matching circuit 71 mountedon the rear glass 1, two FM dynamic matching circuits for use in the FMmain and subsidiary antennas 2 and 4, and the AM dynamic matchingcircuit are integrated on a common substrate. The outputs of one FMdynamic matching circuit and the AM dynamic matching circuit arecombined and then connected to the receiver (not shown) through thetransmission cable 75 of for example 5P3-30BA. The output of theremaining FM dynamic matching circuit is connected to the receiver (notshown) through the transmission cable 76 of for example 1.5C-2V. In thiscase, the FM matching circuits for the FM main antenna and the AMdynamic matching circuit may be FM dynamic matching circuit 30 or 64 andthe AM dynamic matching circuit 50 or 66 respectively as shown in FIGS.12 or 13. The FM matching circuits for the FM subsidiary antenna may beFM dynamic matching circuit 30 or 64 as shown in FIGS. 12 or 13. The FMsubsidiary and/or AM antenna has FM coils 77 and 78 each having forexample 1.6 micro-henry and connected directly to the power buses 72 and73 respectively to provide a monotone change with respect to itsimpedance frequency characteristic in the FM bandwidth. The other end ofthe FM coils 77 and 78 are connected to windings of the choke coil 79 sothat 12 volts DC voltage is applied upon defogging the rear glass 1.

As described above, the present glass antenna system for the automobileof the invention has an integrated circuit mounted on the glass antennaand comprising the FM and AM dynamic matching circuits without anypreamplifier. The FM dynamic matching circuit is active upon receivingthe AM broadcast signal. Its gain characteristic to the FM electric wavehas a narrow selectivity. The probability for reducing the FMinterference electric wave is higher than that of the conventionalreceiving systems having preamplifier. Therefore, the present inventionhas another advantage to attenuate the intermodulation noise invadedupon receiving the AM signal in the area within which two or more ofstrong FM broadcast waves each having strong intensity or electric fieldand their frequency difference entering within the frequency range ofthe AM broadcast.

What is claimed is:
 1. A glass antenna system for an automobilecomprising:two main matching circuits each connected to an antennaprovided on a window glass of the automobile, each of said main matchingcircuits including a first coil and a first varactor diode connectedthereto, to provide first resonance circuits, and two auxiliary matchingcircuits each connected to one of said main matching circuits through acommon node, each of said auxiliary matching circuits including a secondcoil and a second varactor diode connected thereto, to provide secondresonance circuits; a capacitor connected between two of said commonnode; a transmission cable operatively connected to said auxiliarymatching circuits through an output terminal; and resistors eachconnected to said output terminal and each applying a voltage to saidsecond varactor diode through said second coil.
 2. A glass antennasystem for an automobile comprising:a main matching circuit connected toan antenna provided on a window glass of the automobile, said mainmatching circuit including a first coil and a first pair ofcathode-common varactor diodes connected thereto, to provide firstresonance circuits; and an auxiliary matching circuit connected to saidmain matching circuit through a common node, said auxiliary matchingcircuit including a second coil and a second pair of varactor diodeshaving a common cathode connected thereto, to provide second resonancecircuits; a capacitor connected between said common node and ground; atransmission cable operatively connected to said auxiliary matchingcircuit through an output terminal; and resistors each connected betweensaid output terminal and said common cathode.